1. Field of the Invention
The present invention relates to an evaporation apparatus for forming a thin organic film on an object, an organic material evaporation source for use in such an evaporation apparatus, and a method of manufacturing a thin organic film.
2. Description of the Related Art
The conventional electronic material technology has primarily focused on inorganic materials including semiconductors. However, recent years have seen more attention drawn to functional thin organic films of organic compounds as electronic materials.
Organic compounds are advantageous for use as electronic materials because they provide more diverse reactions and properties than inorganic materials and can be surface-treated with lower energy than inorganic materials.
Functional thin organic films are used in organic electroluminescence devices, piezoelectric sensors, pyroelectric sensors, electric insulating films, etc. Electroluminescence devices can be used as display panels, and efforts are being made to develop a technique capable of forming a thin organic film uniformly on a large substrate for producing an electroluminescence display device with a large display area.
Conventional thin organic film fabrication processes employ vacuum evaporation apparatus primarily designed for forming thin metal films including thin films of Al and SiO2, and thin organic films. Evaporation apparatus designed for the fabrication of thin organic films have not yet been developed in the art.
Thin organic film materials have features of their own in contrast to thin inorganic film materials as described below.
Thin organic film materials have high vapor pressures. While metal evaporation sources have an evaporation temperature ranging from 600xc2x0 C. to 2000xc2x0 C., thin organic film materials have a lower evaporation temperature ranging from 0xc2x0 C. (often sub-zero temperatures) to 400xc2x0 C. Many thin organic film materials tend to be decomposed in a temperature range from 20xc2x0 C. to 400xc2x0 C. Therefore, it is preferable to effect precise temperature control for evaporating thin organic film materials.
When a thin metal film is to be fabricated, an electron beam evaporation apparatus is used to apply an electron beam to a metal evaporation source. However, if an electron beam is applied to a thin organic film material, the thin organic film material will be decomposed because the energy of the electron beam is too high for the thin organic film material.
Some thin organic film materials are powdery in nature. Generally, powdery materials have poor thermal conductivity. When a powdery material is heated in a vacuum, its temperature cannot easily be raised or lowered due to the heat insulating effect of the vacuum, and the actual temperature of the powdery material may be delayed with respect to a target temperature at which the powdery material is to be controlled.
Once the temperature of a powdery evaporation source is raised, the powdery evaporation source cannot quickly be cooled by radiation only. Therefore, the material evaporation from the powdery evaporation source is not finished immediately when the heating of the powdery evaporation source is stopped. The material evaporation cannot thus be controlled sharply.
Inasmuch as thin organic film materials have high vapor pressures, absorbs on the wall of a vacuum chamber at a low temperature are likely to be released (reevaporated) as the temperature of the vacuum chamber rises. If such released particles find their way into a thin organic film formed on an object, then they tend to degrade characteristics of the thin organic film.
Many thin organic film materials are capable of easily absorbing moisture. Some of them have their properties modified when they absorb moisture. If moisture is trapped into a multilayer thin organic film as it is formed, then properties of the interlayer boundaries are modified. Such property modifications are liable to result in defects in the final performance of functional devices including electroluminescence devices, piezoelectric sensors, and pyroelectric sensors.
Metal evaporation sources exhibits directivity upon evaporation. The movement of vapor from metal evaporation sources is substantially straight therefrom according to the cosine law. The movement of some thin organic film materials vapor, however, is curved like the direction of particle motion due to diffusion.
For forming an evaporated polymeric film, it is necessary that the composition ratio of two thin organic film materials to be evaporated at the same time be in accordance with a stoichiometric ratio. If the composition ratio differs from a stoichiometric ratio, then a fabricated piezoelectric or pyroelectric device, for example, will lose its functions or suffer function degradation. Film growth speeds need to be precisely controlled for equalizing composition ratio to a stoichiometric ratio.
As described above, thin organic film materials have many properties which make themselves difficult to handle with ease.
FIGS. 8A through 8E of the accompanying drawings show various conventional evaporation sources. These illustrated evaporation sources, however, are not suitable for use with thin organic film materials because of the above properties of thin organic film materials and demanded properties of thin organic films.
FIG. 8A shows a direct resistance heating evaporation source including an evaporation source container 101 of metal which is heated by an electric current passing directly therethrough for evaporating a thin film material.
The direct resistance heating evaporation source provides excellent temperature stability in a temperature range in which metals are melted. However, it has poor temperature stability and controllability in a temperature range in which thin organic film materials are evaporated, with the result that an organic compound vapor (the vapor of a thin organic film material) will be produced at an unstable rate.
Some thin organic film materials have an ability to corrode or react with metals. Those thin organic film materials cannot be used with the evaporation source container 101 which is made of metal.
FIG. 8B shows a conical-basket evaporation source having an evaporation source container 111 and a resistance heater 112 disposed around the evaporation source container 111. When the resistance heater 112 is energized, it indirectly heats a thin film material in the evaporation source container 111 to evaporate the thin film material.
FIG. 8C shows a Knudsen-cell evaporation source having an evaporation source container 121 and a resistance heater 122 disposed around the evaporation source container 121. When the resistance heater 122 is energized, it indirectly heats a thin film material in the evaporation source container 121 to evaporate the thin film material.
Each of the evaporation sources shown in FIGS. 8B and 8C provides excellent temperature stability in a temperature range in which metals are melted. However, it has poor temperature stability and controllability in a temperature range in which thin organic film materials produce vapor, with the result that an organic compound vapor (the vapor of a thin organic film material) will be produced at an unstable rate.
The resistance heater 112 usually comprises a bare metal wire. The movement of thin organic film materials vapor is more likely to be curved than that of thin inorganic film materials. If a thin organic film material contains a metal chelate or the like, it may develop a short circuit between turns of the resistance heater 112.
Because the Knudsen-cell evaporation source is of a complex structure, it cannot easily be cleaned, and the thin film material cannot fully be removed from the Knudsen-cell evaporation source after an evaporation process. Therefore, when the thin film material in the evaporation source container 121 is replaced with another thin film material, the new thin film material may possibly be contaminated with a residue of the previous thin film material.
FIG. 8D shows a lamp-heater type evaporation source comprising an evaporation source container 131 made of a light-transmissive material such as quartz and an infrared lamp 133 disposed above the evaporation source container 131. The infrared lamp 133 applies radiant heat to the evaporation source container 131 to evaporate a thin film material in the evaporation source container 131.
The lamp-heater-type evaporation source has excellent temperature controllability at low temperatures. However, since the evaporation source container 131 has a large specific heat capacity, there tends to be developed a difference between the a target temperature at which the thin film material is to be controlled and an actual temperature of the thin film material. If the temperature of the thin film material is measured for being controlled, a temperature overshooting tends to occur, decomposing a thin organic film material placed in the evaporation source container 131.
The evaporation source container 131 needs to be made of a transparent material such as quartz or the like in order to allow radiant heat emitted from the infrared lamp 133. However, the transparent material is apt to be damaged when it is cleaned or replaced.
After the evaporation source container 131 has been used for a long period of time, it is irregularly fogged and transmits different infrared intensities at different positions. Such different infrared intensities cause a thin organic film material, which has poor thermal conductivity, placed in the evaporation source container 131 to be overheated locally.
Some thin organic film materials are modified by light at a certain wavelength. Such thin organic film materials cannot be evaporated by the lamp-heater evaporation source shown in FIG. 8D.
FIG. 8E shows an electron beam gun evaporation source which applies an electron beam 145 to a thin film material to evaporate the thin film material. The electron beam gun evaporation source shown in FIG. 8E, however, cannot be used to evaporate thin organic film materials because the electron beam 145 decomposes thin organic film materials when applied to them.
As described above, the conventional evaporation sources shown in FIGS. 8A through 8E will suffer various problems if applied to the evaporation of thin organic film materials. Particularly, the decomposition of thin organic film materials due to a temperature overshooting and the difficulty in heating thin organic film materials are problems that have not been experienced with thin inorganic film materials, and should be alleviated.
It is therefore an object of the present invention to provide an evaporation apparatus which is capable of increasing the temperature of an organic film material in a short period of time up to a desired temperature without causing a temperature overshooting and thermally decomposing main constituents of the organic film material.
Another object of the present invention is to provide an evaporation apparatus which is capable of preventing an organic film material from producing a vapor except when the organic film material is evaporated, for thereby effectively exploiting the organic film material from.
Still another object of the present invention is to provide an organic material evaporation source which can uniformly heat a liquid organic film material up to a constant temperature, and allows such a liquid organic film material to be handled with ease.
Yet still another object of the present invention is to provide a method of manufacturing an organic film with such an evaporation apparatus or an organic material evaporation source.
According to the present invention, there is provided an evaporation apparatus comprising a vacuum chamber for holding therein an object on which an organic film is to be formed, an evacuating system connected to the vacuum chamber, for evacuating the vacuum chamber, an organic material evaporation source disposed in the vacuum chamber, for containing an organic film material, and having temperature control means for controlling the temperature of the organic film material to evaporate the organic film material for forming an organic film on the object held in the vacuum chamber, and a gas supply system connected to the vacuum chamber, for introducing an inactive gas into the vacuum chamber.
After the vacuum chamber is evacuated, an inactive gas is introduced into the vacuum chamber to place the organic film material in the organic material evaporation source in an inactive gas atmosphere. When the temperature of the organic film material is controlled, the inactive gas produces a convective flow around the organic film material, with the result that the rate at which the temperature of the organic film material is increased or reduced is higher than if the organic film material were placed in a vacuum atmosphere. If the organic film material is powdery, the inactive gas enters between particles of the powdery organic film material and acts as a heating medium, increasing the heat transfer coefficient between the particles of the powdery organic film material. Therefore, the temperature controllability of the organic film material is increased, preventing the organic film material from suffering localized overheating and a temperature overshooting, so that the organic film material is prevented from being decomposed.
An organic material evaporation source according to the present invention comprises a container for containing a organic film material therein, an outlet port, and an on-off valve connected between the container and the outlet port, the on-off valve being openable for connecting an interior atmosphere in the container to an exterior atmosphere outside of the container through the outlet port, and closable for disconnecting the interior atmosphere in the container from the exterior atmosphere outside of the container.
After an inactive gas is introduced into a vacuum chamber connected to the organic material evaporation source, the on-off valve is closed to place the organic film material in an inactive gas atmosphere.
Generally, an organic compound emits a less vapor in an inactive gas atmosphere than in a vacuum atmosphere. Therefore, when the organic film material is heated and cooled in the inactive gas atmosphere, the emission of a vapor from the organic film material is suppressed during that time. Accordingly, any wasteful vapor that would not contribute to the formation of an organic film is not emitted from the organic film material. The organic film material is thus effectively utilized, and the cost of a manufactured organic film is lowered.
When the organic film material is heated to a temperature at which it would be evaporated in the vacuum atmosphere, the organic film material may be prevented from being evaporated in the inactive gas atmosphere depending on the pressure of the inactive gas atmosphere. Therefore, when the vacuum chamber is evacuated after the temperature of the organic film material is increased in the inactive gas atmosphere, an organic film can be formed without the emission of a wasteful vapor from the organic film material.
The organic material evaporation source further comprises a gas supply system connected to the container, for selectively introducing a gas into the container while the on-off valve is being closed. The organic film material can thus be placed in an inactive atmosphere while the vacuum atmosphere is being developed in the vacuum chamber.
It is therefore not necessary to introduce an inactive gas into the vacuum chamber which is of a large volume in order to suppress the emission of a vapor from the organic film material.
The organic material evaporation source further comprises an evacuating system connected to the container, for selectively evacuating the container while the on-off valve is being closed. The on-off valve can be opened after the container is evacuated to discharge the gas. The organic material evaporation source further comprises a gas supply system connected to the container, for selectively introducing a gas into the container while the on-off valve is being closed.
The organic film material tends to be separated from the vapor when it is cooled. The organic material evaporation source preferably further comprises heating means disposed between the container and the outlet port across the on-off valve, for heating a passage from the container to the outlet port across the on-off valve. The heating means serves to prevent a vapor emitted from the organic film from being cooled until the vapor is discharged from the outlet port into the vacuum chamber.
According to the present invention, an organic material evaporation source comprises a container for containing a liquid organic film material therein, and a heating medium circulatory path for passing a heating medium therethrough, the heating medium circulatory path being disposed around the container. Since the container can be heated or cooled uniformly by the heating medium, the ability for the liquid organic film material to be uniformly heated is increased.
The organic material evaporation source further comprises a heating medium source for controlling the temperature of the heating medium to heat or cool the liquid organic film material contained in the container. Because of a heat exchange between the liquid organic film material and the heating medium, it is possible to increase or reduce the temperature of the liquid organic film material with accuracy. When the temperature of the liquid organic film material is to be increased, since it will not be higher than the temperature of the heating medium, the liquid organic film material is prevented from being locally overheated.
As the liquid organic film material is not heated by heat radiation, its heating will not be made irregular by frosted regions of the container. Because the container is not required to be transparent, it may be made of a ceramic material having a high heat transfer coefficient, and hence can be handled with ease.
The organic material evaporation source further comprises a casing disposed around the container, the heating medium circulatory path being defined between the container and the casing. Inasmuch as a heat exchange occurs between the organic film material and the heating medium through the wall of the container, the efficiency of the heat exchange is high, resulting in increased temperature controllability.
The organic material evaporation source further comprises a heat insulating member disposed around the casing for higher thermal efficiency and temperature controllability.
Alternatively, the organic material evaporation source further comprises a casing disposed around the container, the heating medium circulatory path being defined between the container and the casing, and a heating medium source for controlling the temperature of the heating medium to heat or cool the liquid organic film material contained in the container.
As described above, the rate at which a vapor is emitted from an organic film material varies depending on the pressure of an atmosphere around the organic film material. According to the present invention, there is also provided a method of manufacturing an organic film by emitting a vapor from an organic film material placed in an organic material evaporation source to form an organic film on an object, comprising the step of controlling the pressure of an atmosphere around the organic film material to control the rate at which the vapor is emitted from the organic film material.
According to the present invention, there is further provided a method of manufacturing an organic film by emitting a vapor from an organic film material placed in an organic material evaporation source to form an organic film on an object, comprising the steps of increasing the pressure of an atmosphere around the organic film material to suppress the emission of the vapor from the organic film material when the temperature of the organic film material is increased, and thereafter, reducing the pressure of the atmosphere around the organic film material to start emitting the vapor from the organic film material.
When the pressure of the atmosphere is increased at the time the temperature of the organic film material is increased, the temperature uniformity of the organic film material is increased, shortening the time required to heat the organic film material to increase its temperature up to a desired temperature. When the pressure of the atmosphere is then reduced, the organic film material immediately starts emitting a vapor. Consequently, the time required to form an organic film from the vapor of the organic film material is reduced.
As described above, the organic film material can start and stop emitting a vapor simply by controlling the degree of vacuum of the atmosphere around the organic film material after the temperature of the organic film material is increased. Therefore, any wasteful emission of the vapor of the organic film material which is expensive is prevented.
The step of increasing the pressure of the atmosphere around the organic film material comprises the step of introducing an inactive gas into the atmosphere around the organic film material. The pressure of the atmosphere may be reduced by evacuating the gas.
The method further comprises the step of increasing the pressure of the atmosphere around the organic film material to stop emitting the vapor from the organic film material.
Since the emission of the vapor is immediately stopped, the evaporation of the organic film material can be controlled sharply, and any wasteful emission of the vapor of the organic film material which is expensive is prevented. For cooling the organic film material after an organic film has been formed, the organic film material may be placed in an atmosphere having a high pressure to increase the rate at which the organic film material is cooled.
Experiments have confirmed that when an organic film material is heated to a temperature at which it emits a vapor in a low-pressure atmosphere, the organic film material does not emit a vapor if the pressure of the atmosphere around the organic film material is increased to a range from 13.3 Pa (0.1 Torr) to 2.0xc3x97103 Pa (15.0 Torr) depending on the type of the organic film material.
If the pressure of the atmosphere were too high, the amount of an inactive gas used would be increased, and the time required to evacuate the inactive gas to lower its pressure would also increased. Therefore, it is preferable to set the pressure to 66.5 Pa (0.5 Torr) for suppressing the emission of a vapor.
After the temperature of the organic film material is increase in such a high-pressure atmosphere, the inactive gas is evacuated to develop a low-pressure atmosphere to allow the organic film material to emit a vapor. For increasing the quality of an organic film to be formed, it is preferable to lower the pressure of the atmosphere to 1.33xc3x9710xe2x88x924 Pa (1.0xc3x9710xe2x88x926 Torr) or less, or more preferably to 1.33xc3x9710xe2x88x925 Pa (1.0xc3x9710xe2x88x927 Torr) or less.
The method further comprises the steps of, before the temperature of the organic film material is increased, reducing the pressure of the atmosphere around the organic film material and increasing the temperature of the organic film material to degas the organic film material. To degas the organic film material, it is preferable to heat the organic film material to a temperature lower than its evaporation temperature.
A method according to claim 13, wherein said step of increasing the pressure of the atmosphere around said organic film material comprises the step of introducing an inactive gas into the atmosphere around said organic film material.
A method according to claim 13, wherein said step of increasing the pressure of the atmosphere around said organic film material comprises the step of introducing an inactive gas into the atmosphere around said organic film material; and
comprising the step of reducing the temperature of said organic film material.